This invention relates to a method for forming a concentric layer of controlled thickness on the interior of a cylindrical body. This invention further relates to an apparatus for forming the said layer.
Generally used methods for application of polymeric ablative materials in ramjet combustors are by transfer molding, back extrusion (plunge casting), vacuum filling, and spin casting. The first three methods require a core-forming insert or mandrel that molds or shapes the inside surface of the insulating ablative material. Spin casting does not require an insert or mandrel. Instead, the inside surface is formed by centrifugal force, concentric with the spin axis.
Transfer molding involves filling the annulus between the cylindrical body and the core-forming insert or mandrel with the ablative material by pumping or by transfer from a pressurized vessel. The annulus may often have a width of 0.25 inch or less. Extreme care must be exercised to avoid entrapment of air or other gas in the material during such filling. The most significant problem with injection molding into a narrow annulus is alignment of reinforcing fibers, admixed with the polymeric material, parallel to the forming surfaces. In-plane alignment becomes a major problem because the fibers do not provide the radial reinforcement required for char stability. In-plane fibers allow axial fissures and also promote stratification of char layers, allowing them to slough.
Back extrusion or plunge casting comprises forcing a mandrel into a mold containing a pool of uncured material. The uncured material is forced into the thus-formed annulus in a manner similar to compression molding. Precise alignment of the mandrel and the mold is necessary to produce concentricity and uniformity of the uncured material. Undesirable fiber alignment results because of wall drag; fibers are oriented axially rather than concentrically.
Vacuum filling is similar, in terms of material transfer, to transfer molding. A mandrel is used, but instead of forcing the polymeric material into the annulus, the annulus is evacuated to a low pressure, and atmospheric pressure then forces the uncured resin into the annulus. This method has the same fiber alignment problems as transfer molding.
Spin casting differs from the above-described processes in that an insert or mandrel is not used. The cylindrical body or mold is spun while an uncured resin is deposited along the inside wall of the mold. The rate of spinning, and the resultant centrifugal force causes the uncured resin to flow axially to level the surface. An excessive spinning rate can cause segregation of the resin and any fillers and/or reinforcing fibers admixed therewith. Leveling of the resin at reasonable speeds, i.e., speeds which will not cause segregation, is somewhat limited. Variations of 0.010 to 0.020 inch or more have been observed over an axial distance as small as 6 inches.
Various spin casting techniques have been employed to introduce an uncured compound to the inside of the cylindrical mold. Early methods involved pouring the uncured material into the cylinder while stationary, revolving the cylinder slowly to distribute the mass, and finally, spinning rapidly to level the surface. This method is not highly successful because the uncured fluid may drip or fold over itself during the slow spinning step, entrapping air in the mass. Further, circumferential and axial leveling are often less than required, resulting in the need to machine the surface.
Slow speed spinning with a doctor blade has been attempted to distribute the uncured material. Air entrapment occurs because the blade causes the fluid material to fold over itself during spinning. The slow speed of revolution necessary in order to use the doctor blade does not provide sufficient centrifugal force to deaerate the viscous mass. Moreover, it is difficult to remove the doctor blade without disturbing the uncured polymeric surface.
Current spin casting methods are not fast enough or economical for high rate production. Also, concave surfaces, precisely defined tapered surfaces and the like, are difficult to form using current methods.
Accordingly, it is an object of the present invention to provide an improved method for spin casting.
It is another object of the present invention to provide a novel apparatus for spin casting.
Other objects and advantages of the present invention will be apparent to those skilled in the art from a consideration of the following description and the attached drawings.